Move suitesparse-specific operators to .ss namespace (and deprecate current location) #364

SuiteSparse-specific operators such as any_firsti that can't be recreated using UDFs should be moved to ss namespace and their current location deprecated.

For example, use gb.semiring.ss.any_firsti instead of gb.semiring.any_firsti (which should warn upon use).

This will help python-graphblas support other GraphBLAS implementations in the future.